WASHINGTON — In December 2015, delegates from 195 nations announced to a roar of applause in Paris that they had finally sealed a pact aimed at keeping global temperatures from rising more than 2 degrees Celsius to avoid the worst ravages of climate change.

The negotiators knew even then that this goal would be extremely difficult to meet under the terms of the accord. But now that President Trump has announced that the United States will withdraw from the Paris climate agreement, it seems all but impossible.

“Even before the United States pulled out, no country was in the ballpark of doing what needed to be done,” said Glen Peters, a researcher at the Center for International Climate and Environmental Research in Norway, who has warned that the world is unlikely to keep global warming below 2 degrees.

In the wake of Mr. Trump’s withdrawal, leaders in Europe, China and elsewhere are insisting they will carry on tackling global warming anyway.

Miguel Arias Cañete, the European commissioner for climate action and energy, denounced Mr. Trump’s decision, brandishing a copy of the agreement as he spoke to reporters in Brussels on Friday. “The fight against climate change cannot depend on the result of elections in one country or another,” he said.

But what’s now very much in question is whether the world can still meet its broader climate goals. The Earth has already warmed 1 degree since humans began burning fossil fuels, and countries have dithered so long in taking action that little short of acrash program to choke off the flow of carbon dioxide into the atmosphere would keep the temperature rise well below 2 degrees.

Analyses by Mr. Peters and others have found that the United States, Europe and other wealthy nations would need to sharply accelerate their efforts and shift to a near-zero-carbon economy by midcentury. That would include steps like phasing out coal plants and gasoline-burning cars within mere decades. Poorer countries like China and India would need to quickly follow, even as they faced the task of lifting millions out of poverty.

Under the Paris deal, countries submitted voluntary pledges for curbing emissions that, various analyses have found, would put the world on pace for 3 degrees or more of warming — an improvement over doing nothing, but still far short of what is necessary. The hope was that countries would collectively ratchet up their efforts, slowly inching toward the deep cuts needed to stave off 2 degrees.

The United States, which accounts for one-fifth of the world’s emissions, had slowly been making headway on that task as cheaper natural gas and renewables drove hundreds of coal plants into retirement. As part of the Paris agreement, the Obama administration put forward a pledge to cut domestic emissions 26 to 28 percent by 2025. And, in November, the White House put out a report sketching out various technological pathways to cutting emissions 80 percent or more by 2050.

“We put 28 percent reductions by 2025 as a goal because you could still draw a plausible path from there to deep decarbonization by midcentury,” said Andrew Light, a senior climate change adviser at the State Department under President Barack Obama. “If we don’t hit that, the whole game plan becomes much harder.”

Yet even if Hillary Clinton had become president, the 2025 pledge would have proved difficult to meet. Although emissions are falling in America’s electricity sector, pollution from cars, trucks and heavy industry remains stubbornly tough to suppress. Changing that, the Obama administration argued, would require a flurry of new policies to cut emissions and a doubling of federal research funding into newer, cleaner technologies like advanced batteries.

Mr. Trump has vowed to dismantle Mr. Obama’s regulations on greenhouse gases and slash energy research budgets. Even if states tried to pick up some of the slack, a recent analysis from the Rhodium Group, an economic research company, found that emissions would fall only 15 to 19 percent.

Experts say it is still possible for the United States to hit its Paris targets. The cost of battery-powered electric vehicles could drop faster than expected, leading to rapid shifts in the transportation sector. Solar and wind power could outstrip all expectations. Cities, states and private corporations could step up their own efforts to purchase renewable energy, expand mass transit and improve the energy efficiency of their buildings. Michael R. Bloomberg, the former mayor of New York, has so far corralled 30 mayors, three governors and 100 businesses into pledging to help the United States meet its Paris pledge despite Mr. Trump.

But that outcome is far from assured, and experts say that deeper emissions cuts past 2025 will prove only more difficult unless policies for a far-reaching energy transition are laid down today.

Other nations could try to fill the void left by Mr. Trump, though they face barriers to doing so. China’s emissions have been leveling off far earlier than expected as it cancels plans for new coal plants and invests heavily in cleaner sources like solar, wind and nuclear. Yet even that would be only a first step: Many projections for staying below 2 degrees envision China’s emissions not just flattening out but falling sharply starting around 2030 or so, which would require the rapid closing of hundreds of coal plants or outfitting them with still-costly technology to capture and bury their carbon dioxide underground.

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Miguel Arias Cañete, the European commissioner for climate action and energy, in Madrid last month. In Brussels on Friday, he denounced President Trump’s decision to withdraw from the Paris climate agreement.CreditChema Moya/European Pressphoto Agency

The European Union, for its part, has vowed to cut emissions 40 percent below 1990 levels by 2030. But it will not be easy for Europe to pursue even deeper cuts in the short term to compensate for American inaction, since coal-reliant countries like Poland have resisted stricter curbs on emissions. Meanwhile, Germany is struggling to meet its Paris pledges as it works to shutter its remaining nuclear power plants.

For now, world leaders are unwilling to give up the 2-degree goal, although Mr. Peters says that detailed scenarios for staying below those levels increasingly envision the use of technology to suck carbon dioxide out of the air on a scale that may ultimately prove unrealistic.

While scientists are careful to stress that 2 degrees is not a bright line between climate safety and danger, they do note that as temperatures pass soar past that mark, the risks increase significantly, including the destabilization of ice sheets in Greenland and Antarctica, rising sea-levels, more destructive heat waves and droughts, and the loss of vital ecosystems like coral reefs.

“At some point it may become hard to deny that 2 degrees is unavoidable,” Mr. Peters said. When that happens, he said, “the world will have to have a difficult conversation about what to do next.”

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Meeting Climate Goals Was Always Hard. Without the U.S., It’s Far Harder.. Order Reprints | Today’s Paper | Subscribe